Asus P6T Deluxe v2 6/4GB Nightmare

[El Tel]

Spec
M/B: Asus P6T Deluxe V2
CPU: Intel Core i7 750
RAM: G.Skill Trident F3-12800CL6T-6GBTD
O/S Windows 7 64Bit

Really need some help here - i've got no more hair left to pull out.
On every 1st Boot the bios detects 6GB -CPU-Z registers the RAM in slots 1,3,5 (which is correct),
but on every subsequent reboot bios detects 4GB - CPU-Z registers RAM in slots 1,2,3 (which is incorrect)

it makes no difference if the Computer is cold or warmed up the results are the same.
i've read a lot about similar problems in various forums and as such have checked the CPU pins which seem ok
It's not a CPU cooler too tight issue because i've even tried testing with the cooler not even screwed in place and the result was exactly the same.
I don't know my way around the bios too well with regards to memory settings but nothing i've tried has improved the situation in there,
currently i've underclocked the ram to DDR800 and left everything else on auto and STILL no improvement.
I'm not looking to overclock anything i just want 6Gb + stability which i can't find atm, i'm open to any suggestions

[GSKILL TECH]

Make sure the memory is firmly inserted. Sometimes one side may be slightly sticking up and that can cause the system to read 4GB at times. If you continue to have the problem, simply send the memory in for new replacements.

Thank you
GSKILL SUPPORT

[El Tel]

Yep turns out it was a bad DIMM, couldn't get consistant results with bios setting at auto but when i turned it up to XMP settings i was getting consistant errors in memtest with one DIMM whilst the other two were ok no matter which slot i used.
Hopefully i can RMA only the faulty DIMM and keep the two good ones here so I can still use the computer